The present invention relates to an adhesive composition considerably reduced in residual stress, and to the utilization of this composition. More particularly, it relates to; an adhesive composition that can be used to bond two or more kinds of members which maintain high airtightness and are very different from each other in terms of their thermal coefficient of expansion, a composite member comprising members bonded using the adhesive composition, and a method for producing the composite member using the adhesive composition.
In the case of bonding of different kinds of members, especially when one of the members is easily damaged by thermal stress, cracks occur at and around the bonded interface during the bonding operation, particularly during the cooling operation after bonding at high temperatures, and the desired bond strength cannot be maintained. Therefore, the resulting composite member, which is a bonded body of the different members, is frequently broken during use causing accidents and other related problems. Furthermore, since cracks occur in the member which is weak against thermal stress, high airtightness as composite members cannot be maintained, and hence, the composite members must be disposed of as defective products, which results in an overall increase in costs.
For example, as an example of bonding of different members, a brazing material is used for the bonding of a ceramics member and a metallic member. In this case, in order to assure wetting of the ceramics member with the brazing material, generally after the surface of the ceramics member to be bonded is plated with a metal such as Ni, both the members are positioned facing each other with a suitable space in between and then a brazing material is poured into the space to bond the members. However, this method is not sufficient for diminishing thermal stress, and cracks often occur on the side of the ceramics member which is easily damaged by thermal stress, to affect not only the bond strength, but also various performances required for composite members, such as airtightness.
Furthermore, in order to ease the thermal stress, there is employed a method of using as an intermediate material a metal of low thermal coefficient of expansion or a soft metal capable of easing thermal stress upon plastic deformation at the time of bonding and a method of optimizing the interfacial reaction layer.
However, these techniques also have faults and are not necessarily highly flexible. There is a high-pressure solid phase bonding method which is now under development, but this method has not yet attained sufficient bonding strength and still has some unresolved issues which must be cleared up before practical use is possible.
On the other hand, as a composite solder, JP-A-6-126479 discloses a solder containing a powder comprising a material with a higher melting point than a solder. By filling the powder, comprising a material with a higher melting point than the solder, only into the central portion of the body of solder, this composite solder aims at overcoming the insufficient wetting with solder due to the powder present on the surface of conventional solders. In other words, this composite soldier aims at increasing the bond strength at the bonded interface. However, this composite solder does not effectively diminish thermal stress, and thus cannot be used for the bonding of different kinds of members which contain large differences in the amount of strength that they possess against thermal stress.
The present invention provides, an adhesive composition for bonding two or more kinds of different members, which can bond the kinds of different members and maintain a proper bonding strength without causing a phenomenon, due to thermal stress during the cooling operation after bonding at high temperatures, of reduction in bond strength at or around the bonded interface, or the occurrence of cracks at the members which are easily damaged by thermal stress during cooling operation; a composite member comprised of members bonded using the adhesive composition; and a method for producing the composite member using the adhesive composition.
As a result of intensive research conducted by the inventors, it has been found that the above objects can be attained by using a brazing material as a base, which is less restricted by kind or shape of the members to be bonded, and has a wide range of selections for bonding form, and by adding a particulate material capable of reducing thermal stress to said brazing material. Thus, the present invention has been accomplished.